The aviation industry relies upon numerous navigation aids in order to safely take off, navigate enroute, and land aircraft. Such navigation aids (navaids) include, for example, the instrument landing system (ILS), very high frequency omni-directional range (VOR) system, and the like. The survey industry also relied upon numerous location aids in order to ensure the most precise measurements are being performed. The Navstar Global Positioning System, hereafter referred to simply as GPS, is increasingly being accepted as an alternative to traditional navigation and survey aids. In addition to civilian applications, GPS is used extensively by the United States Department of Defense (DoD) to provide military users with highly accurate position, velocity, and time information.
GPS is a space-based radio positioning network for providing users equipped with suitable receivers highly accurate position, velocity, and time (PVT) information. Developed by the United States Department of Defense (DoD), the space-based portion of GPS comprises a constellation of GPS satellites in non-geosynchronous 12-hour orbits around the earth.
Prior art FIG. 1 shows the constellation 100 of GPS satellites 101 in orbit. The GPS satellites 101 are located in six orbital planes 102 with four of the GPS satellites 101 in each plane, plus a number of “on orbit” spare satellites (not shown) for redundancy. The orbital planes 102 of the GPS satellites 101 have an inclination of 55 degrees relative to the equator and an altitude of approximately 20,200 km (10,900 miles) and typically complete an orbit in approximately 12 hours. The positions of GPS satellites 101 are such that a minimum of five of the GPS satellites 101 are normally observable (above the horizon) by a user anywhere on earth at any given time.
GPS position determination is based upon a concept referred to as time of arrival (TOA). ranging. Each of the orbiting GPS satellites 101 broadcasts spread spectrum microwave signals encoded with positioning data and satellite ephemeris information. The signals are broadcast on two frequencies, L1 at 1575.42 MHz and L2 at 1227.60 MHz, modulated using bi-phase shift keying techniques. Essentially, the signals are broadcast at precisely known times and at precisely known intervals. The signals are encoded with their precise time of transmission. A user receives the signals with a GPS receiver designed to measure the time of arrival of the signals and to demodulate the satellite orbital data contained in the signals. Using the time of arrival data from multiple satellites, and multiplying this by the speed of light gives what is termed the pseudo range distance measurement to the various satellites. If the GPS receiver clock were perfect, this would be the range measurement for that satellite, but the imperfection of the clock causes it to differ by the time offset between actual time and receiver time. Thus, the measurement is called a pseudo range, rather than a range. However, the time offset is common to the pseudo range measurements of all the satellites. By determining the pseudo ranges of four or more satellites, the GPS receiver is able to determine its location in three dimensions, as well as the time offset. Thus, a user equipped with a proper GPS receiver is able to determine his PVT with great accuracy, and use this information to navigate safely and accurately from point to point, among such other uses as survey and mapping and vehicle tracking. For more information on how the GPS system works, see (Parkinson & Spilker).
DoD GPS applications require the most accurate PVT possible, which is obtained via encrypted P-code signals of the PPS. These applications also need to be secure from jamming, spoofing and other types of countermeasures. As is well known, PPS is a high accuracy (e.g., published specifications to 6 meters 2 DRMS horizontal, Circular Error Probable CEP, or 16 meters Spherical Error Probable SEP) service used by DoD authorized users (e.g., the military). PPS is based upon processing p-code signals modulated on both the L1 frequency and the L2 frequency. When encrypted, the P-code becomes known as Y-code, and must be decrypted after demodulation, necessitating the use of special crypto key equipment available only to DoD authorized users using specialized GPS receiver equipment.
The GPS system was conceived for military use first and foremost, but later was designated a dual-use system by President Ronald Reagan in 1983. That is because the military system, known as the precise Positioning System (PPS) uses a 10.23 MHz data rate for the coded signal, referred to as the Precise Code or P-code, while the Coarse Acquisition code, called the C/A code is generally used as an aid to acquire the P-code. The accuracy of the position fix from P-code is approximately lo times that available to un-enhanced C/A code civilian receivers. Since the introduction of commercial receivers in 1984 by Trimble Navigation Limited, the enhancements introduced for civilians have vastly outpaced those introduced for military applications. The military receivers have improved for their specific applications, but there are few if any PPS-based survey, mapping, or tracking applications incorporated in the SPS receivers.
The needs of military receivers and the needs of civilian receivers are thus quite different, and have never been integrated before. Thus there is a need to incorporate the features of various civilian receivers with military receivers so that the benefits of Commercial Off The Shelf (COTS) procurement practices can be obtained in equipment that meet military needs.
Prior Art FIG. 2 shows a typical prior art PPS receiver system 200. System 200 shows the specialized encryption receiver components utilized in generating the encrypted Y-code signal. As is well known in the art, a replica of the Y-code signal must be generated by a GPS receiver in order to acquire and track the Y-code signal transmitted from the respective GPS satellites. System 200 depicts the components required to generate the Y-code signal replica.
As shown in FIG. 2, system 200 includes a P-code generator 201 coupled to a Y-code generator 202 via line 204. Y-code generator 202 is coupled to a security module (e.g., PPS-SM, SAASM, or M-code engine) 208 via line 207. KDP 208 is also coupled to a CV 205 (crypto-variable) keying device and a computer system 211 via line 206 and line 209.
System 200 functions by generating a Y-code replica for use by an incorporating GPS receiver in locking onto a transmitted Y-code signal from a GPS satellite. As is well known, a Y-code is generated by properly encrypting the,P-code. P-code generator 201 generates a replica P-code. and couples this P-code to Y-code generator 202 via line 204. Y-code generator 202 encrypts this P-code using a data received from security module 208 via line 207. Y-code generator 202 generates the Y-code 210 by encrypting the P-code. The Y-code 210 is coupled to a DSP 220 where it is used to process Y-code signals received from the GPS satellites via antenna 222 and RF front end 221. The resulting positioning information is subsequently coupled to the computer system 211 via line 223. Security module 208 also couples Selective Availability (SA) data to computer system 211 via line 209 which allows the computer system 211 to cancel out the PVT errors due to selective availability (SA).
The security module 208 functions by generating the data used by Y-code generator 202. As is known by those skilled in the art, security module 208 generates the data by using a CV (crypto-variable) key 205. Thus, system 200 enables the incorporating GPS receiver to decode and process the encrypted Y-code signals from the GPS constellation.
Only users equipped with GPS receivers which incorporate Y-code hardware (e.g., security module 208, and Y-code generator 202) and which have current CV keys are able to process the Y-code signals. Consequently, access to the CV keys are very tightly controlled. In addition, the design of the encrypting hardware (security modules) is very tightly controlled. This high level of control greatly increases the cost of fielding and maintaining an inventory of PPS receivers.
In addition, current security module are typically implemented as chip sets of three or more discrete integrated circuits. Accordingly, the security module accounts for a significant portion of the cost of the PPS receiver. The multi chip security module implementation also increases the complexity of a PPS receiver, and the like. These are all disadvantages when the objective is to use highly accurate and cost effective PPS receivers in the military, especially in the case of disposable PPS receivers for use with PGMs.
Due to the complexity and associated cost of the PPS receiver, the technology of the PPS receiver is years behind that of the civilian SPS receiver. Therefore, most advances with GPS based technology occurs on the civilian (SPS) side of the GPS market. Due to the dissimilar advances in GPS technology, the SPS receiver is more technologically (e.g., software and hardware) advanced than that of the PPS receiver. That is, the SPS receiver may contain newer technology, require less power to function, and be able to operate more advanced software than that of the PPS receiver. However, the cost associated with developing many of the civilian features available for non-military users into PPS receivers is quite prohibitive, and would never lead to any cost effective solutions.
What is needed is a method for easily integrating SPS receiver systems with PPS receiver technology in a cooperative operation. What is also needed is a method for SPS and PPS cooperative operation which allows the better applications of the civilian SPS receiver to operate in the more challenged DoD environment of the PPS receiver. What is further required is a method which provides these advantages without compromising accuracy, integrity, or security. The present invention provides a novel solution to the above requirements.